This week is the beginning of a new season in the CW superhero universe, starting with The Flash and Arrowseason premieres before the CW debut of Supergirl and the return of the Legends of Tomorrow. Things will culminate with all four shows crossing over in early December, yet they have already gotten a head start by training together in "Superhero Fight Club 2.0"

The first of many CW hero crossovers in the next several months premiered on the CW app, but has now finally made its way to YouTube right before the new seasons begin.

Supergirl and the Flash already met when Grant Gustin raced onto Supergirl late last season, leaving Oliver Queen less equipped to deal with both Barry Allen and Kara Danvers' quirky senses of humor. Whether or not that will also apply when they all meet in a real TV episode, at least Oliver and Barry are on the same wavelength in being annoyed with the even quirkier Ray Palmer/The Atom.

While they are joined by White Canary and Firestorm on the training floor, Felicity and Cisco are on overwatch duties up above, with an added assist from the music of Guns n Roses. And although Diggle is actually in battle on the other side of the world when Arrow Season 5 starts, canon is rewritten to give him an early stunned peek at Hank Henshaw's Martian Manhunter form here.

Four minutes of heroes from four different shows fighting together is one thing, but having them come together for four straight nights of full episodes two months from now will be another. Ironically, these crossover episodes are filming right now, with various on set pictures coming in from Melissa Benoist, Caity Lotz, Stephen Amell and others.

It might be a headache keeping all these heroes in line, yet perhaps no more so than the challenges in rewriting an entire extended universe. That is how this season is starting in the CW hero world, thanks to The Flash changing history and setting off ripple effects across Arrow and other shows with its "Flashpoint" arc.

Time will fittingly tell if the Flashpoint universe will still stand by the time the Flash, Arrow, Supergirl and the Legends meet again. But the countdown to that reunion will begin when The Flash resumes on Tuesday, Oct. 4 at 8 p.m. est on the CW, followed by the Arrow premiere 24 hours later, the Supergirl CW premiere with Superman on Monday, Oct. 10 and the Legends of Tomorrow returning on Thursday, Oct.13.